Astrology is a pure science. Predictions is applied astrology. The science involves mathematical calculation for the precise location of planets and the signs of Zodiac at a given instance. The astrological chart (like the birth chart) is basically a chart of the heavens at a particular time. The sky above is divided into twelve sections also know as the twelve houses. At any particular time one of the house or that slice of the sky is rising at the eastern horizon which makes it the rising sign or the ascendant. The various planets are placed in there respective astronomical place in the chart.
'Predictions' are the accurate deduction of the calculations, involving techniques under the presentsocialandenvironmentalconditions. Naturally, the practitioner must be well experienced, knowledgeable, wise and impartial because Knowledge, Experience and ability to translate the ancient text to current situations goes into correct and practical interpretation of this chart.
Since ancient time astrology has been used by Kings and kinsmen for guidance. And basically that is what the function of astrology is. It is a guiding science and not a binding science. It's guidance can help you make wise decisions and reduce the impact of unhappy events.